A fiber laser apparatus is excellent in light collecting properties, provides a small beam spot of high power density, and can be manufactured in a non-contact manner, so that the fiber apparatus is used in various fields such as a laser beam machining field and a medical field. The fiber laser apparatus for use particularly in the machining field and the medical field provides high output.
An optical fiber amplifier is used in the fiber laser apparatus. In the optical fiber amplifier, a pumping light is input to an amplification optical fiber doped with an active element such as a rare earth element, and the input pumping light is absorbed in the active element. Thus, the active element is caused in an excited state, a light to be amplified is amplified by the stimulated emission of the active element in the excited state, and the light is output. Therefore, desirably, the pumping light input to the amplification optical fiber is absorbed in the active element with no waste. In order to cause the active element to absorb the pumping light with no waste as described above, it is efficient to prolong the amplification optical fiber. However, when the amplification optical fiber as prolonged, such a region is produced in which the ratio of an active element not caused in the excited state by the pumping light is high, and the light to be amplified and the amplified light tend to be absorbed in the active element in this region. As a result, in this optical fiber amplifier, although the pumping light is absorbed, with no waste, the conversion efficiency of the light to be amplified into the output light is degraded.
Therefore, in an optical fiber amplifier described in Patent Document 1 below, in order to efficiently absorb a pumping light, an optical multiplexer/demultiplexer separates a pumping light, which enters an amplification optical fiber but is not absorbed in the amplification optical fiber, from a signal light, and the separated pumping light is reflected off a total reflection film, and again enters the amplification optical fiber. As described above, the pumping light that once goes out from the amplification optical fiber is again caused to enter the amplification optical fiber, and thus the pumping light is efficiently absorbed in the active element of the amplification optical fiber for efficient amplification.